Electric machines having pulse-controlled inverters are used in hybrid motor vehicles, for example, where they are operated optionally in motor operation or generator operation. In motor operation, the electric machine generates an additional driving torque, which supports the internal combustion engine, e.g., in an acceleration phase; in generator operation, it generates electric power, which is stored in an energy accumulator, e.g., a battery or a supercap. The operating mode and power of the electric machine are adjusted via the pulse-controlled inverter.
Conventional pulse-controlled inverters (PCI) include a number of switches with which the individual phases (U, V, W) of the electric machine are optionally switched to a high potential, the so-called d.c. link voltage, or to a low reference potential, in particular ground. The switches are triggered by an external control unit, which calculates a setpoint operating point for the electric machine as a function of the driver input (accelerate or brake). The pulse-controlled inverter is connected to the control unit and receives the corresponding operating data and/or control commands from it.
In the case of a malfunction which may occur, e.g., due to an excessively high battery current or an excessively high feeder current, the pulse-controlled inverter is switched to a safe state to prevent possible damage to electric components. Generally, there are two different conventional shutdown methods that are used here alternatively. In a first method, all switches connected to the low potential (low-side switches) are closed and all switches connected to the high potential (high-side switches) are open. This operating mode is also referred to as short-circuit mode. In another shutdown method, all the switches of the pulse-controlled inverter are open. This is also referred to as the disconnect mode.
The conventional shutdown methods are suitable in principle for protecting the energy accumulator and/or electric components connected thereto, but they have some disadvantages. For example, the phase current may increase for a short time (1-1.5 half waves) after switching to the short-circuit mode. The deployment threshold for the short-circuit mode must therefore be set low accordingly. In addition, switching to the short-circuit mode causes a sudden change in the machine torque, which may have an interfering effect on vehicle performance. Furthermore, in the short-circuit mode, a substantial electric power loss occurs in the electric machine and may result in overheating. The disconnect mode, however, has the disadvantage in particular that the d.c. link voltage may increase by 50 V or more after switching to the disconnect mode. The energy accumulator and other electric components may be overloaded and/or damaged.